Apparatus for guiding medical tools have been shown to be of valuable assistance in various medical procedures, for example, manipulation of surgical tools, manipulation of cameras or sensors, biopsy, etc. An apparatus for guiding a medical tool usually also improves reproducibility compared to freehand medical procedures, for example, surgical or biopsy procedures.
These apparatus typically have one or more degrees of freedom and may be manually driven through these degrees of freedom with motive force being provided by a human practitioner, or may be automated in that at least one degree of freedom is driven by a computer controlled actuator. A medical tool often needs to be oriented about a point in, on, or in proximity to a patient's body. These apparatus can also monitor the position of the medical tool in a frame of reference such that the position of the tool can be identified in, for example, a medical image associated with the frame of reference. These apparatus are sometimes referred to as trackers.
Generally, trackers include one or more linkages that move relative to one another to position a medical tool relative to the patient. Most of the linkages are constrained to a single degree of freedom and thereby allow for precision measurement of their relative movement. Calculations (e.g. geometric calculations) use the relative movement between the linkages along with the lengths of the linkages to calculate the position of a free end of the tracker (end of the last linkage) in three-dimensional space. The last linkage typically supports the medical tool and often allows movement of at least two degrees of freedom. That is, the last linkage often allows for rotational movement and axial advancement or retraction of a medical tool. In the case of a TRUS ultrasound probe, this allows the probe to be advanced/retracted relative to a patient and rotated to acquire images. Therefore, the movement of the last linkage must be tracked in two dimensions. The provision of two degrees of freedom in this linkage complicates position sensing for this linkage and hence position sensing for the medical tool.